1. Field of the Invention
The present invention relates to a method for one-side root pass welding of a pipe joint in a pipe-laying work.
2. Description of the Prior Art
A groove shape in a circumferential butt weld of a pipe joint is generally formed as shown in FIG. 1 (A). The pipe joint is welded as shown in FIG. 1 (B). That is, root pass welding (I), hot pass welding (H), filler pass welding (F) and cap pass welding (C) of the pipe joint are carried out in this order. Firstly, the root pass welding (I) is carried out by the use of an internal welding machine. A welding speed of the root pass welding (I) is about 75 cm/min and weaving of a torch is not carried out. Then, after the root pass welding (I) has been carried out, the hot pass welding (H), filler pass welding (F) and cap pass welding (C) are carried out in this order from an outer surface of pipes with the use of an external welding machine. The welding speed of the hot pass welding (H) is about 100 cm/min and the weaving of the torch is not carried out. The welding speed of the filler pass welding (F) and the cap pass welding (C) is about 30 to 40 cm/min and the pipe joint is welded while the weaving of 6 to 7 mm corresponding to a width of the groove is being carried out.
Since any weaving of a torch is not carried out during welding of the pipe joint by the root pass welding and the hot pass welding in the above-described prior art, a seam tracking control of welding cannot be carried out with the use of arc sensors. Accordingly, time and labour are required to adapt a welding torch of a welder to the center of a groove of the pipe joint. Moreover, defects are liable to occur in a bead of the root and hot pass due to such factors as a propensity of wire to bend and errors of the torch in pointing at positions.
Welding current is limited to about 200 A in the prior art root pass welding. Because a shape of a bead becomes convexed particularly in a position of the torch pointing upward at the time of using electric current higher than 200 A since any wearing of a welding torch cannot be applied. Accordingly, welding of the pipe joint has been regarded as impossible at a higher speed.
An automatic seam tracking control by the use of the arc sensors is carried out as follows:
(a) Make the welding torch weave in the groove in the direction of the groove width. PA0 (b) Detect the deviation of the welding torch by detecting the waveforms of welding current and arc voltage. PA0 (c) Track the welding seam automatically by correcting the torch position on the basis of the information of (b). PA0 forming a ring groove having a V-shaped bottom portion thereof at end faces of both pipes whose joint is circumferentially welded; PA0 attaching a backing material to a butt portion of the pipes from an internal side of the pipes; PA0 running an automatic welding machine along a guide rail mounted on an outer surface of the pipes in the circumferential direction thereof; PA0 controlling a position of a welding torch along a seam by means of an arc sensor; and PA0 welding root pass from the outer side of the pipes with a predetermined welding current and at a welding speed in a high-speed rotating arc welding of the pipes at a predetermined rotational speed and with a predetermined diameter of rotation.
Since a vibrating weaving method is adopted in the prior art arc sensors, it is difficult for the weaving frequency to exceed 10 Hz because of mechanical limits such as vibration, backlash or the like. Due to the limitation of the weaving frequency, a welding speed applicable from a viewpoint of responsiveness of the arc sensors and shapes of beads is limited. The automatic seam tracking control cannot be applied at a welding speed of 75 cm/min in the prior art root pass welding.